1. Field of the Invention
This invention relates to an apparatus for recording and reproducing information on and from a recording medium such as an optical disk, and in particular to an apparatus for recording and reproducing information on and from a recording medium which can accurately and stably control the rotational speed of the recording medium.
2. Description of the Background of the Invention
An apparatus for recording and reproducing on and from an optical disk, an apparatus using a magnetooptical disk, an apparatus using a write-once type optical disk, and an apparatus using a phase-transfer type optical disk are known. In all of these apparatus, information is recorded on or reproduced from an optical disk along tracks which are formed concentrically or spirally on the disk, while rotating the optical disk.
For example, the apparatus for recording and reproducing from a magnetooptical disk uses a magnetooptical disk as a recording medium in which the recording portion consists of a magnetic film having the axis of easy magnetization perpendicular to the film face. When recording information on the disk, an energy beam such as a laser beam focused to a diameter of about 1 .mu.m is irradiated on the film to raise the temperature of a region of the film along a track, and the magnetization of the heated region is reversed by applying an external magnetic field. When information is to be reproduced from the disk, using photomagnetic effect between the linear polarlization of a laser beam and the magnetic film, a laser beam is scanned on the disk, and the change in the polarized direction of the light reflected from or transmitted through the magnetic film is analyzed by an analyzer, and converted to a series of electrical signals.
In such an apparatus, the rotational speed of the magnetooptical disk is controlled so that the linear velocity of the region irradiated by the laser beam is kept constant (hereinafter, this control is referred to as "CLV control"). Conventionally, the CLV control is conducted by either of the two control processes described below.
In the first control process, the rotational speed of the magnetooptical disk is detected by a rotation frequency generator such as a tachometer or rotary encoder attached to a spindle motor which drives the magnetooptical disk. The radial position of the irradiated region (hereinafter, referred to as "radial tracking position") is detected by a position sensor such as an optical encoder or potentiometer attached to an optical head which receives the reflected or transmitted light. The objective rotational speed is determined from the detected radial tracking position. Then, the control of the rotational speed of the disk is conducted by taking the feedback difference between the detected rotational speed and the objective rotational speed. The first control process is characterized in that the CLV control is conducted by obtaining the objective rotational speed from the detected radial tracking position.
In the second control process, a magnetooptical disk is used in which ordinary signals for recording information as well as mark signals such as clock pulses were recorded under a constant predetermined linear velocity. When information is reproduced from the disk, both the mark signals and information signals are reproduced, and the frequency or phase relation of the reproduced mark signals are detected from the detected signals to obtain the linear velocity of the irradiated region. Then, the control of the rotational speed of the disk is conducted by taking the feedback difference between the detected rotational speed and the objective rotational speed. For example, in the CLV control for a so-called compact disk, the linear velocity is detected from a frame synchronizing signal which are included in the reproduced signals.
An apparatus for recording and reproducing on and from an optical disk of the prior art in which information can be recorded, reproduced and erased by either of the aforementioned control processes has a drawback that the reliability of the reproduced signals deteriorates because the recording portion of such an optical disk consists of a part of the recording portion (hereinafter, referred as "recorded region") where information has been already recorded, and the other part of the recording portion (hereinafter, referred as "non-recorded region") where information has not yet been recorded. This will be described in more detail below.
When a laser beam is scanned on a recorded region, according to the first control process, the output signal of the rotary frequency generator and the detected information signals are different from each other in frequency or phase, resulting in a variation of the frequency or phase of the reproduced signal. Therefore, the first control process cannot accurately control the linear velocity of the region to be scanned. In contrast, the second control process can perform a reliable control of the linear velocity of the region to be scanned because the control is effected on the basis of the signals detected from the mark signals.
When a laser beam is scanned on the non-recorded region, the first control process can control accurately and stably the linear velocity of the non-recorded region because the first control process does not use mark signals. By contrast, in the second control process, no information or data required for controlling the linear velocity can be obtained, resulting in a runaway of the spindle motor.
Hence, a prior art apparatus for recording and reproducing information on and from an optical disk such as a magnetooptical disk, in which apparatus either of the two control processes is conducted, cannot accurately and stably control the linear velocity of a region to be scanned so that the reliability of the reproduced signals is lowered.
The timing control for recording, reproducing or erasing information may be conducted as described below. As shown in FIG. 13, a regenerative circuit 31 supplies a reproduced signal A to a sector timing detection circuit 32. In the sector timing detection circuit 32, for example, synchronization detection can be performed at each sector, for example, using a signal such as a sector mark signal which is included in the reproduced signal A. A synchronization detection signal B from the sector timing detection circuit 32 is supplied to a timing control circuit 33 in which the timing control is performed on the basis of the synchronization detection signal B. When reproducing information, a reproduction timing signal D is sent to the regenerative circuit 31, and when recording or erasing information, a recording/erasing timing signal E is sent to a recording/erasing circuit 34. The sector timing detection circuit 32 conducts the synchronization detection separately from the reproduction of information so that the synchronization detection is not disturbed by the reproduction timing signal D.
In the apparatus shown in FIG. 13, the timing control is performed on the basis of only the synchronization detection signal B. If the synchronization detection is erroneously conducted, or if a time lag in detection is produced in the sector timing detection circuit 32, the timing control cannot be performed accurately. For example, the timing control is disturbed in such a case that a read gate signal of a PLL (Phase Locked Loop) becomes inaccurate, that a capturing action becomes impossible, or that the unlocking easily occurs. Such an apparatus for recording and reproducing information from an optical disk has drawbacks that the reliability in the recording, reproduction or erasing of information is lowered, and that the timing control such as an AGC (Automatic Gain Control) is hindered.